1. Field of the Invention
The present invention relates to a sheet feed roller that is used for a printing apparatus, such as a printer, to appropriately carry sheets, such as recording papers, inserted between a pressure roller and the sheet feed roller, and to a method of manufacturing the same.
2. Description of the Related Art
As shown in FIG. 10, the conventional sheet feed roller 21 includes a cylindrical metal roller portion 22. On the circumferential surface of the roller portion 22, a plurality of projections with a predetermined height 23 is formed at predetermined intervals in the circumferential direction and the axial direction of the roller portion 22.
In such a conventional sheet feed roller 21, a pressure roller 24 is elastically forced against the circumferential surface of the roller portion 22 by a coil spring (not shown), and a sheet 25, such as a recording paper having a predetermined thickness, is inserted and pressed between the roller portion 22 and the pressure roller 24.
In this state, when the sheet feed roller 21 is rotated in the forward or reverse direction, the projections 23 grip the sheet 25 to reliably reciprocate the sheet 25 in a direction perpendicular to the printable surface of the paper.
When printing the desired image on the sheet 25, the sheet 25 is fed into a printing portion of a printing apparatus (not shown) by the rotation of the sheet feed roller 21, so that the desired image can be printed.
According to a method of manufacturing the projections 23, as shown in FIG. 11, a pair of punches 27 is mounted to a holder 26 so as to be opposite to each other. The gap between the pair of punches 27 is smaller than the diameter of the roller portion 22.
In addition, the sheet feed roller 21 is rotatably supported by a V-shaped supporting stand 28.
By repeatedly performing a punching operation in which the punches 27 raised to a raised position at a predetermined height are dropped to a position shown in FIG. 11, and a rotating operation in which the roller 21 is sequentially rotated by a predetermined angle in synchronism with the raising of the punches 27 to the raised position after the punching operation, a straight grain projection 23a is formed by the punch 27 on the right side of FIG. 11, and a reverse grain projection 23b is formed by the punch 27 on the left side of FIG. 11.
As shown in FIG. 13, the projections 23 are formed such that the pitch between adjacent straight grain projections 23a in the axial direction (in the horizontal direction of FIG. 13) is P and that the reverse grain projections 23b are formed between the straight grain projections 23a in the circumferential direction, that is, in the vertical direction of FIG. 13.
Furthermore, the rotation angle α formed between adjacent straight grain projections 23a in the circumferential direction is 6°, and the reverse grain projections 23b are formed between the straight grain projections 23a formed at the rotation angle of 6° in the circumferential direction and are also formed at a distance of P/2 from the straight grain projections 23a in the axial direction.
That is, as shown in FIG. 13, the projections 23 are formed in a zigzag shape along the circumferential direction and the axial direction on the circumferential surface of the roller portion 22.
When the conventional sheet feed roller 21 having the above configuration is used for a printing apparatus, capable of performing color printing, such as a thermal transfer printer, the plurality of projections 23 grips both surfaces of the sheet 25, such as thick photographic paper. As a result, the sheet 25 is gripped and is carried reciprocatively. An ink layer of an ink ribbon (not shown) is thermally transferred to the reciprocating sheet 25, thereby printing the desired color image on the sheet 25.
According to the conventional sheet feed roller 21 having the aforementioned configuration, a grip force on the sheet 25 while it is being carried can be increased by changing the height of the projections 23 according to the thickness of the sheet 25, and thus the sheet 25 can be reliably carried.
[Patent Document 1]
Japanese Patent No. 3271048 (corresponding U.S. Pat. No. 6,532,661)
Japanese Patent No. 3352602
Japanese Unexamined Patent Application Publication No. 10-119374
However, as shown in FIG. 12, when the rotation angle α formed between adjacent straight grain projections 23a in the circumferential direction is, for example, 6° and the height of the straight grain projections 23a is increased, the punches 27 dropped according to the punching operation may interfere with the previously formed straight grain projections 23a to cut the tops of the previously formed straight grain projections 23a. 
Therefore, the plurality of projections 23 must have the height at which the punches 27 do not interfere therewith during the punching operation, or the rotation angle α must be increased. As a result, the number of projections 23 gripping the sheet 25 per unit area is decreased, and thus the grip force on the sheet 25 is decreased.